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Measuring current using a Shunt Resistor & an Oscilloscope

FernandoDAS

New Member
Hi people, I'm new using forums for this kind of purposes, so please forgive me if I can't explain myself well.

I have to measure the output current & voltage of a High Frecuency, High Voltage transformer, I can measure the Output Voltage using a High Voltage Probe and my Handheld Oscilloscope, but I still have to deal with the current, I want to use a Homemade Shunt Resistor (0.1ohms) and my Oscilloscope, but before doing it I tried by measuring that voltage with my Fluke Multimeter, I was increasing the input voltage & It was working well but at some point my Multimeter Screen started to fade & show random numbers, What happened? The voltage on the Shunt wasn't that high (around 2.5 volts) when it started to fail, it still works. I don't want to damage my Oscilloscope, so I need to now what causes that issue.

Here is my circuit diagram if you wan't to know the way I'm measuring.

Considerations:
-I know my multimeter doesn't measure at those frequencies, but the error is constant so I can just adjust it.
-I know my oscilloscope does work at those frequencies.
-I have no idea of the value of the output current & voltage, also by measuring another transformer the values are not consistent.
 

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dr pepper

Well-Known Member
Most Helpful Member
Your meter might not be happy with the noise from the shunt, the leakage inductance of the transformer will resonate with any capacitance connected to it, and this could be high in voltage, more than the meter expects.
Also its not a good idea to measure high freq's on a meter that isnt meant for that kind of measurement.
 

Reloadron

Well-Known Member
Most Helpful Member
You are showing a 20 KHz source square wave voltage. Your meter reflects a 1000:1 high voltage probe acting as a divider. Both your meter and the probe need to have an upper frequency limit of 20 KHZ and since you reflect a square wave source voltage your transformer needs to be able to operate at 20 KHz. Finally the meter used needs to be a true RMS responding and indicating meter and not one of the more common average responding RMS indicating type meters. Also to use the shunt you would measure the shunt voltage drop directly across the shunt which I don't see and I also don't see a load out there?
 

unclejed613

Well-Known Member
Most Helpful Member
you haven't shown what the load is on the high voltage side. to measure current your shunt resistor will be connected between the low side of the load and ground. you haven't said what the "high voltage" is, so be aware that most multimeters can be damaged by voltages above 400V. the meter will have that information printed on it near whewre the probes connect, and may say the max voltage is 400V (usually 400V for a cheap meter, but may be as high as 2000V for a really good one). you don't want to exceed the max voltage printed on the meter. you will need a "True RMS" meter to measure the output accurately, and the measurement bandwidth of the meter needs to be >20khz. most cheap meters only measure average voltage, and "fudge factor" it for the readout, and most meters only have a measurement bandwidth between 500hz-1khz.

oscilloscopes also have a maximum voltage that can be measured, usually around 1000V, and that should be marked somewhere near the input connector.
 

FernandoDAS

New Member
Your meter might not be happy with the noise from the shunt, the leakage inductance of the transformer will resonate with any capacitance connected to it, and this could be high in voltage, more than the meter expects.
Also its not a good idea to measure high freq's on a meter that isnt meant for that kind of measurement.
Hi, & thank you for replying.
Mmmhhh I never thought about the resonance, pretty nice observation.
About the meter.... I'm stuck with it, I have no access to a proper one.
 

FernandoDAS

New Member
You are showing a 20 KHz source square wave voltage. Your meter reflects a 1000:1 high voltage probe acting as a divider. Both your meter and the probe need to have an upper frequency limit of 20 KHZ and since you reflect a square wave source voltage your transformer needs to be able to operate at 20 KHz. Finally the meter used needs to be a true RMS responding and indicating meter and not one of the more common average responding RMS indicating type meters. Also to use the shunt you would measure the shunt voltage drop directly across the shunt which I don't see and I also don't see a load out there?

Hi, & thank you for replying.
Before anything, are you the same user "Reloadron" from "All About Circuits", if so, I answered some things there.
There is no load, only the probe & the shunt.
I'm measuring the shunt voltage drop on the terminal seen in the picture & the ground.
 

FernandoDAS

New Member
you haven't shown what the load is on the high voltage side. to measure current your shunt resistor will be connected between the low side of the load and ground. you haven't said what the "high voltage" is, so be aware that most multimeters can be damaged by voltages above 400V. the meter will have that information printed on it near whewre the probes connect, and may say the max voltage is 400V (usually 400V for a cheap meter, but may be as high as 2000V for a really good one). you don't want to exceed the max voltage printed on the meter. you will need a "True RMS" meter to measure the output accurately, and the measurement bandwidth of the meter needs to be >20khz. most cheap meters only measure average voltage, and "fudge factor" it for the readout, and most meters only have a measurement bandwidth between 500hz-1khz.

oscilloscopes also have a maximum voltage that can be measured, usually around 1000V, and that should be marked somewhere near the input connector.
Hi, & thank you for replying.
There's no load on the high voltage side, only the probe.
The voltage across the shunt must be around the 2-3 Volts (only a guess, but it cannot be too high).
The output voltage is reduced by 1000, so I see no problem for measuring it (I'm doing this using a Fluke 123 Handhelp Oscilloscope).
 

Reloadron

Well-Known Member
Most Helpful Member
Hi, & thank you for replying.
Before anything, are you the same user "Reloadron" from "All About Circuits", if so, I answered some things there.
There is no load, only the probe & the shunt.
I'm measuring the shunt voltage drop on the terminal seen in the picture & the ground.
Yes, that would be me, one in the same. I just left you a new comment in the thread over there. :)

Ron
 

unclejed613

Well-Known Member
Most Helpful Member
The output voltage is reduced by 1000, so I see no problem for measuring it (I'm doing this using a Fluke 123 Handhelp Oscilloscope).
make sure then that there's a ground reference for the high voltage side then, the maximum voltage isolation (float voltage) is 750Vrms. i once saw somebody fry a very expensive Beckman multimeter (back when Beckman prices were "if you gotta ask, you can't afford it") by measuring across a voltage source which was only 100V or so, but was floating at 1500V. the meter display began flashing random gobbeldygoop. fortunately, Beckman replaced the meter under warranty, but with a "don't let that happen again" note with the new meter. the maximum float (or "common mode") voltage is as important as the max voltage across the meter
 

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